Labels serve two purposes, really. The first being a form of organization: when I want to find a certain message, I can find it properly filed away under it's appropriate label.

The second is prioritization and "action markers." This is where labels aren't the optimal solution. Because the labels don't truly prioritize anything (your inbox doesn't get sorted by priority, except for "priority" and "everything else")--and it's a loosely ruled system--you need to define rules that result in actions to actually create priorities.

I think that's where Mailbox app will shine: it enforces a priority system. Whether or not it will eliminate the need for proper categorization for other purposes, though? I'm doubtful.